Dual-stack optical data storage medium and use of such medium

ABSTRACT

A dual-stack optical data storage medium ( 10 ) for recording using a focused radiation beam ( 9 ) having a wavelength λ is described The beam enters through an entrance face ( 8 ) of the medium ( 10 ) during recording. The medium comprises at least one substrate ( 1, 7 ) with present on a side thereof a first recording stack ( 2 ) named L 0 , comprising a recordable type L 0  recording layer ( 3 ), said first recording stack L 0  having an optical reflection value R L0  and an optical absorption value A L0  at the wavelength λ, and a second recording stack ( 5 ) named L 1  comprising a recordable type L 1  recording layer ( 6 ), said second recording stack L 1  having an optical reflection value R L1  and an optical absorption value A L1  at the wavelength λ, and a transparent spacer layer ( 4 ) sandwiched between the recording stacks ( 2, 5 ). The second recording stack is present closer to the entrance face than the first recording stack. By fulfilling the formula A L1 ≦1−R min −√(R min /R L0 ) in which formula R min  is the minimum required effective optical reflection value for each recording stack full compatibility is achieved with a read only (ROM) version of the medium.

The invention relates to a dual-stack optical data storage medium forrecording using a focused radiation beam having a wavelength λ andentering through an entrance face of the medium during recording,comprising:

at least one substrate with present on a side thereof:

a first recording stack named L₀, comprising a recordable type L₀recording layer, said first recording stack L₀ having an opticalreflection value R_(L0) and an optical absorption value A_(L0) at thewavelength λ,

a second recording stack named L₁ comprising a recordable type L₁recording layer, said second recording stack L₁ having an opticalreflection value R_(L1) and an optical absorption value A_(L1) at thewavelength λ, said second recording stack being present closer to theentrance face than the first recording stack,

a transparent spacer layer sandwiched between the recording stacks, saidtransparent spacer layer having a thickness substantially larger thanthe depth of focus of the focused radiation beam.

The invention also relates to the use of such a medium.

An embodiment of an optical recording medium as described in the openingparagraph is known from Japanese Patent Application JP-11066622.

Regarding the market for optical recording, it is clear that the mostimportant and successful format so far is a write-once format, CompactDisk Recordable (CD-R). Although the take-over in importance by CompactDisk ReWritable (CD-RW) has been predicted since a long time, the actualmarket size of CD-R media is still at least an order of magnitude largerthan for CD-RW. Furthermore the most important parameter for drives isthe maximum write speed for R-media, not for RW. Of course, a possibleshift of the market to CD-RW is still possible, e.g. because of MountRainier standardization for CD-RW. However, the R-format has been provenvery attractive due to its 100% compatibility with read only compactdisk (CD).

Recently the Digital Versatile Disk (DVD) has gained market share as amedium with a much higher data storage capacity than the CD. Presently,this format is available in a read only (ROM) and a rewritable MW)version. Next to the DVD ReWritable (DVD+RW) standard a new recordable(R), i.e. write once, DVD+R standard was developed. The new DVD+Rstandard gets increasing attention as an important support for DVD+RW. Apossible scenario is that the end customers have become so familiar withan optical write-once format that they might accept it more easily thana re-writable format. Recently a new format has been introduced calledBlu-ray Disc (BD) with even a higher storage capacity. For this formatalso R and RW versions will be introduced.

An issue for both the R and RW formats is the limited capacity andtherefore recording time because only single-stacked media are present.Note that for DVD-Video, which is a ROM disk, dual layer media alreadyhave a considerable market share. A dual-layer, i.e. dual-stack, DVD+RWdisk is probably feasible. However, it has become clear that a fullycompatible disk, i.e. within the reflection and modulation specificationof the dual-layer DVD-ROM, is very difficult to achieve and requires atleast a major breakthrough for the properties of theamorphous/crystalline phase-change materials, which are used asrecording layers in e.g. DVD+RW media Without a full compatibility, thesuccess of a dual-layer DVD+RW in the market is questionable.

In order to obtain e.g. a dual-stack DVD+R medium which is compatiblewith the dual-layer (=stack) DVD-ROM standard, the effectivereflectivity of both the upper L₁ layer and the lower L₀ layer should beat least 18%. More generally it can be said that any new generation dualstack medium requires a minimum effective optical reflection levelR_(min) in order to meet a specification, e.g. for a dual stack BD theexpected value of R_(min) is 0.04 and for a dual stack BD compatiblewith a single stack BD R_(min)=0.12. Effective optical reflection meansthat the reflection is measured as the portion of effective light comingback from the medium when e.g. both stacks L₀ and L₁ are present andfocusing on L₀ and L₁ respectively. The conditions, which must beimposed on the optical reflection, absorption and transmission values ofthe stacks in order to meet such a specification are by far not trivial.In JP-11066622 nothing is mentioned about requirements with respect tooptical reflection, absorption and transmission values of the stacks. Itshould be noted that in this document the normally used convention ofnotation of L₀ and L₁, in which notation L₀ is the “closest” stack, i.e.closest to the radiation beam entrance face, has been changed: L₀ now isthe deepest stack, as seen from the radiation beam entrance face, and L₁is the stack closer to the radiation beam entrance face.

It is an object of the invention to provide an optical data storagemedium of the type mentioned in the opening paragraph which has aneffective optical reflection level of both the L₀ stack and the L₁ ofmore than a specified value R_(min).

This object has been achieved in accordance with the invention by anoptical storage medium, which is characterized in thatA_(L1)≦1−R_(min)−√(R_(min)/R_(L0)) in which formula R_(min) is theminimum required optical effective reflection value for each recordingstack. For a given optical data storage medium, the effective reflectionof both recording stacks of a dual-stack disc, should always lie above aspecified minimum reflection R_(min). This implies that the effectivereflection of L₁ should meet the following criterion:R _(L1eff) =R _(L1) ≧R _(min)  Eq(1)For L₀, the effective reflection should beR _(L0eff) =R _(L0) *T _(L1) ² ≧R _(min)  Eq(2)Thus, we obtain a requirement for the transmission of L₁ ofT _(L1)≧√(R _(min) /R _(L0))  Eq(3)Equations (1) and (3) show that the optical properties of the totaldual-stack medium are mainly defined by the optical properties of L₁.The combination of equations (1) and (3) directly defines a requirementfor the allowable absorption of L₁:A _(L1)≦1−R _(min)−√(R _(min) /R _(L0))  Eq(4)The maximum A_(L1) that is ever allowable is obtained for maximumreflection of L₀, i.e. when R_(L0)=1. In this case, also the highestpossible effective reflection from L₀ is obtained. Thus we can define amaximum for the absorption in L₁ that is still allowed as follows:A _(L1max)=1−R _(min)−√(R _(min))  Eq(5)The choice R_(L0)=1 implies that it is impossible to write data into L₀since no absorption of optical radiation occurs. This extreme situationwould e.g. be applicable to a dual-stack recordable-ROM disc orrecordable L₁, ROM L₀ disc.

In an embodiment A_(L1)≦A_(L0). In order to be able to recordinformation via optical means in L₀, the L₀ stack should have a finiteoptical absorption at the wavelength of the radiation beam, e.g. alaser. Since only part of the light of the recording laser istransmitted through L₁, L₀ should preferably be made more sensitive,i.e. have a higher absorption than L₁, in order to keep the requiredwrite-power within acceptable limits. For a recordable dual-layer discit seems natural to impose the following two conditions: (i) sameeffective reflection of both layers (same signal amplitudes which ispreferred from drive point-of-view) and (ii) same effective absorptionin both layers (same write-powers needed irrespective of layer). Thesetwo boundary conditions give rise to a preferred absorption in L₁ thatis given by:A _(1pref)=1−3 R _(min)/4−¼−¼·[(1−R _(min))²+8R _(min)]^(1/2)  Eq(6)Then, the preferred absorption in L₀ (assuming T_(L0)=0) is given byA _(0pref)=1−R _(min)/{¼−R _(min)/4+¼·[(1−R _(min))²+8R_(min)]^(1/2)}²  Eq(7)The next step is to recognize that the absorption in L₀ and L₁ is mainlydetermined by the thickness of the recording layer d_(L) in L₀ and L₁respectively and the absorption coefficient k_(Lλ) of the recordinglayer material in L₀ and L₁ respectively (k_(Lλ) is the imaginary partof the complex refractive index n_(Lλ)). To estimate the absorptionwithin the recording stack the effect of a possible dual-layer stackdesign is omitted, which implies the following simplifications: (i)interference effects within the recording layer are neglected, (ii)possible absorption in additional layers that may be present isneglected, (iii) recording layer is embedded in between twosemi-infinite media having complex refractive index n0 and n2, see FIG.5. Typically the upper surrounding medium will be transparent (substratefor L₁ and spacer for L₀) while the lower medium will be eithertransparent (spacer for L₁) or highly reflecting (mirror for L₀). Then,the absorption of optical power within this layer depends exponentiallyon both d_(L) and k_(L) and is calculated to be: $\begin{matrix}{A = {\left\lbrack {1 - {\exp\left\lbrack {\frac{{- 4}\pi\quad d_{L}k_{L}}{\lambda}\left\lbrack {1 + \left( {\frac{n_{L} - n_{2}}{n_{L} + n_{2}}} \right)} \right\rbrack} \right\rbrack}} \right\rbrack \cdot \left\lbrack {1 - \left( {\frac{n_{0} - n_{L}}{n_{0} + n_{L}}} \right)^{2}} \right\rbrack}} & {{Eq}(8)}\end{matrix}$λ is the wavelength of the laser. The term (1+|(n_(L)−n₂)/(n_(L)+n₂)|)in the exponent is a measure for the effective thickness increase due tothe portion of light that is reflected back at the second interface ofthe recording layer, see FIG. 5. The multiplication-factor(1−|(n_(L)−n₀)/(n_(L)+n₀)|²) accounts for the light that is reflected atthe first interface. Typically, the L₁ stack will be tuned for bothfinite reflection and transmission. Then, the most dominant contributionto the stack's absorption will be the absorption for a single-pass oflight. The L₀ stack will be tuned for high reflection, and the stack'sabsorption will be close that for a double-pass of light.

Preferably 1.5A_(L1)≦A_(L0)≦2.5A_(L1). From FIG. 4 it can be seen thatfor equal write-power in L₀ and L₁, the absorption in L₀ shouldtypically be approximately twice that of L₁. For the range of mostinterest, i.e. finite absorption to achieve high T in L₁ and high R inL₀, the double pass will yield approximately twice as much absorption.Thus, in order to have the absorption of both layers in the requiredrange, the following is valid for both layers:0.5*A _(L0max) ≈A _(L1max)=1−R _(min)−√(R _(min))≦1−exp(−4πk _(L) d_(L)/λ)  Eq(9)From FIG. 6 it can be seen that this approximation is best for the L₁type stacks, where of course interference effects play a less importantrole.

One effect that is not taken into account in the above calculations isthe presence of the guide grooves in the medium, which are normallypresent for tracking purposes in each recording stack adjacent therecording layer. Due to these grooves, the radiation beam is diffractedand only a part (or none) of the diffracted light is captured by thereflection/transmission measurement setup. Thus the diffraction appearslike a kind of absorption. The diffraction is used to generate trackingsignals like push-pull and track-cross and preferably these signals areof equal magnitude on both stacks to minimize adjustments to theservo-systems when switching between the stacks. This in turn means thatfor both layers a similar amount of light is lost in thereflection/transmission measurement. It means that the indicated rangesof absorption and k/d range are really the upper-limit that is allowedsince the range is derived assuming no diffraction losses at all.

In an embodiment, for the recordable type L₁ recording layer having acomplex refractive index ñ_(L1λ)=n_(L1λ)−i*k_(L1λ) at the wavelength λand having a thickness d_(L1), the following formula is fulfilled:

k_(L1λ)≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π*d_(L1)) in which formulak_(L1λ) is the absorption coefficient of the L₁ recording layer.

In a further embodiment, for the recordable type L₀ recording layerhaving a complex refractive index ñ_(L0λ)=n_(L0λ)−i*k_(L0λ) at thewavelength λ and having a thickness d_(L0), the following formula isfulfilled:

k_(L0λ)≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π*d_(L0)) in which formulak_(L0λ) is the absorption coefficient of the L₀ recording layer.

It is noted that the above analysis is more accurate for low k-valuesk<1); for k>1 the presented formula becomes inaccurate although it stillcan serve as a rough estimate. Further it should be noted that thedefinition of the thickness d_(L0) and d_(L1) of the recording layersrequires some further explanation. It may e.g. be that the recordinglayer thickness in a guide groove is different from the thickness inbetween guide grooves due to leveling effects during the application ofthe recording layer by e.g. spincoating. Hence the thickness of therecording layer is defined as being the thickness where the focusedradiation beam spot is present during recording and read out.

To obtain a recordable dual-stack optical data storage medium that meetsthe specifications of the dual-layer (stack) DVD-ROM disc, it isrequired that λ is about 655 nm, R_(min)=0.18 and that k_(L0λ) andk_(L1λ) fulfil the the requirements of equations Eq(9) and Eq(10).

To obtain a recordable dual-stack optical data storage medium that meetsthe (expected) specifications of the dual-stack Blu-ray Disc (BD), it isrequired that λ is about 405 nm, R_(min)=0.04 and that k_(L0λ) andk_(L1λ) fulfil the the requirements of equations Eq(9) and Eq(10).

For a dual-stack Blu-ray Disc which is compatible with the single layerreflection specification, it is required that X is about 405 nm,R_(min)=0.12 and that k_(L0λ) and k_(L1λ) fulfil the requirements ofequations Eq(9) and Eq(10). Preferably 0.7*d_(L0)<d_(L1)<1.3*d_(L0) forthe media described the last three paragraphs.

It should be noted that the invention is not limited to a single sideddual stack medium but that by varying substrate thicknesses e.g. twosingle sided dual stack media according to the invention may be bondedtogether forming a dual sided dual stack medium, which fulfils thicknessrequirements.

The invention will be elucidated in greater detail with reference to theaccompanying drawings, in which

FIG. 1 shows a schematic layout of a dual-stack optical data storagemedium according to the invention. The effective reflection of bothstacks is indicated.

FIG. 2 shows the maximally allowable absorption in L₁ as a function ofthe imposed minimum effective reflectivity of both layers of thedual-stack disc.,

FIG. 3 shows the preferred absorption in L₀ and L₁ compared to maximallyallowable absorption in L₁ as a function of the effective reflectivityof L₀ and L₁.

FIG. 4 shows the ratio between optical absorption in L₀ and L₁ as afunction of effective reflection.

FIG. 5 shows a schematic layout of the absorption of an opticalradiation beam by an absorbing recording layer, neglecting interferenceeffects within the recording layer.

FIG. 6 shows a comparison between calculated absorption andapproximation of Eq (9) for L₁ type of stack (left) and L₀ type of stack(right). Solid line: exact calculation; dashed line: approximation.

FIG. 7 shows the maximum value of allowed k versus L₁-recording layerthickness for various values of effective reflection in case of a laserwavelength within the DVD specification.

FIG. 8 shows the range of allowed k-values as a function of L₁-recordinglayer thickness for a dual-stack medium that meets the DVDspecifications (laser wavelength 655 nm, Rmin=18%).

FIG. 9 shows the maximally allowed k-value in the case of aDVD-compatible (for R=18%) and a BD-compatible (for R=4%) dual-layerdisc.

In FIG. 1 a dual-stack optical data storage medium 10 for recordingusing a focused radiation beam, e.g. a laser beam 9, having a wavelengthλ is shown. The laser beam enters through an entrance face 8 of themedium 10 during recording. The medium 10 comprises substrates 1 and 7with present on a side thereof a first recording stack 2 named L₀,having an optical reflection value R_(L0) and an optical absorptionvalue A_(L0) at the wavelength λ and a second recording stack 5 named L₁having an optical reflection value R_(L1) and an optical absorptionvalue A_(L1) at the wavelength λ,

a transparent spacer layer 4 is sandwiched between the recording stacks2 and 5, said transparent spacer layer 4 having a thickness of 50 μmwhich is substantially larger than the depth of focus of the focusedlaser beam 9. The absorption value fulfils the following equation:

A_(L1)≦1−R_(min)−√(R_(min)/R_(L0)) in which formula R_(min) is theminimum required effective optical reflection value for each recordingstack.

The first recording stack 2, comprises a recordable type L₀ recordinglayer 3, e.g. an azo dye or any other suitable dye. A guide groove ispresent in the first substrate 1 or in the spacer layer 4, a firsthighly reflective layer is present between the L₀ recording layer 3 andthe substrate 1. A second substrate 7 is present with on a side thereofa second recording stack 5 comprising a recordable type L₁ recordinglayer 6, e.g. an azo dye or any other suitable dye. The second L₁recording stack 5 is present at a position closer to the entrance face 8than the L₀ recording stack 2. A second guide groove is present in thesecond substrate 7 or in the spacer layer 4. The first substrate 1 withL₀ is attached to the substrate with L₁ by means of the transparentspacer layer 4, which may act as bonding layer. Specific suitable L₀/L₁stacks designs are described below.

Embodiment 1 DVD Recordable Dual Stack R_(min)=0.18, λ=655 nm, (Layersin this Order):

-   -   Substrate 1 made of PC having a thickness of 0.60 mm    -   Reflective layer of 100 nm Ag (n=0.16-5.34i), Au, Cu or Al, or        alloys thereof, may be used as well,    -   L₀ recording layer 3 of an azo dye, with thickness of 80 nm, the        refractive index of the dye at a radiation beam wavelength of        655 nm is 2.24-0.02i.    -   First semitransparent reflective layer made of Ag having a        thickness of 10 nm, Au, Cu or Al, or alloys may be used as well,    -   Spacer layer 4 made of a transparent UV curable resin having        having a thickness of 50 μm,    -   Second semitransparent reflective layer made of Ag having a        thickness of 10 nm, Au, Cu or Al, or alloys may be used as well,    -   L₁ recording layer 6 of an azo dye, with thickness of 80 nm, the        refractive index of the dye at a radiation beam wavelength of        655 nm is 2.24-0.02i.    -   Substrate 7 made of PC having a thickness of 0.58 mm        This stack design has the following reflection, absorption and        transmission values:        A_(L0)=0.4        A_(L1)=0.2        R_(L0)=0.6        R_(L1)=0.2        T_(L1)=0.6        T_(L0)=0        The formula        A_(L1)≦1−R_(min)−√(R_(min)/R_(L0))=1−0.18−√(0.18/0.6)=0.27 has        been fulfilled. Furthermore k_(L0λ)*d_(L0)=1.6        nm≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π)=26.4 nm and        k_(L1λ)*d_(L1)=1.6 nm≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π)=26.4        nm.

The first semitransparent reflective layer may also be a SiO₂ layer witha thickness of e.g. 20 nm; other dielectrics may be used as well. In adifferent embodiment the first semitransparent reflective layer may beabsent. Furthermore, additional dielectric layers may be present betweenthe recording layer and the reflective and/or semitransparent reflectivelayers. The second semitransparent may also be a dielectric (e.g. SiO2)or semiconducting (e.g. Si) layer. Furthermore, additional dielectriclayers may be present between the recording layer and the secondsemitransparent reflective layer and/or between second semitransparentreflective layer and the spacer layer and/or between the recording layerand the substrate 7.

Embodiment 2 BD Recordable Dual Stack R_(min)=0.12, λ=405 nm (Layers inthis Order):

-   -   Substrate 1 made of PC having a thickness of 1.1 mm    -   Reflective layer of 100 nm Ag (n=0.17-2i), Au, Cu or Al, may be        used as well,    -   L₀ recording layer 3 of an organic dye, with thickness of 50 nm,        the refractive index of the dye at a radiation beam wavelength        of 405 nm is 2.4-0.04i.    -   First transparent dielectric layer made of SiO2 having a        thickness of 20 nm, other dielectrics (Si3N4, ZnS—SiO2, Al2O3,        AlN) may be used as well,    -   Spacer layer 4 made of a transparent UV curable resin having a        thickness of 25 μm,    -   L₁ recording layer 6 of an organic dye, with thickness of 50 nm,        the refractive index of the dye at a radiation beam wavelength        of 405 nm is 2.4-0.04i.    -   Second transparent dielectric layer made of SiO2 having a        thickness of 20 nm, other dielectrics (Si3N4, ZnS—SiO2, Al2O3,        AlN) may be used as well.    -   Substrate 7, in this embodiment also called cover layer, made of        a transparent UV curable resin, having a thickness of 0.075 mm.        This stack design has the following reflection, absorption and        transmission values:        A_(L0)=0.6        A_(L1)=0.2        R_(L0)=0.4        R_(L1)=0.2        T_(L1)=0.6        T_(L0)=0        The formula        A_(L1)≦1−R_(min)−√(R_(min)/R_(L0))=1−0.12−√(0.12/0.4)=0.33 has        been fulfilled. Furthermore k_(L0λ)*d_(L0)=2        nm≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π)=24 nm and        k_(L1λ)*d_(L1)=2 nm≦{λ*ln[1/(R_(min)+√(R_(min)))]}/(4π)=24 nm

In FIG. 2 a graph is drawn representing the maximum allowable absorptionin L₁ A_(L1max) as a function of a minimum effective reflection R_(min)of both recording stacks L₀ and L₁. Note that the maximum achievablevalue of R_(min) is about 0.38. This value represents the case in whichthe L₁ stack does not have absorbance anymore and hence recording is notpossible, while also the L₀ stack has no absorption and maximumreflection (R_(L0)=1).

In FIG. 3 the preferred absorption in L₀ and L₁ are compared to themaximally allowable absorption in L₁ as a function of the effectivereflectivity of L₀ and L₁. This preferred absorption graphs arerepresentations of equations (6) and (7).

In FIG. 4 the ratio between A_(L0) and A_(L1) is shown as a function ofthe effective reflectivity of L₀ and L₁. It can be seen that preferablythis ratio is in the range 1.5-2.5 more preferably in the range 1.5-2.0.

In FIG. 5 a schematic layout of a recording layer 3, 6 in a dual stackoptical data storage medium 10 is shown (see FIG. 1). The path of anoptical radiation beam is show. The absorption in L₀ and L₁ is mainlydetermined by the thickness of the recording layer d_(L) and theabsorption coefficient k_(Lλ) of the recording layer material (k_(Lλ) isthe imaginary part of the complex refractive index n_(Lλ)). To estimatethe absorption within the recording stack the detailed effect of apossible dual-layer stack design is omitted, which implies the followingsimplifications: (i) interference effects within the recording layer areneglected, (ii) possible absorption in additional layers that may bepresent is neglected, (iii) recording layer is embedded in between twosemi-infinite media having complex refractive index n0 and n2.Typically, the upper surrounding medium will be transparent (substratefor L₁ and spacer for L₀) while the lower medium will be eithertransparent (spacer for L₁) or highly reflecting (mirror for L₀). Then,the absorption of the optical radiation beam within this layer dependsexponentially on both d_(L) and k_(L), represented by equation (8).

In FIG. 6 modeling results are presented of the absorption as a functionof the recording layer thickness. The solid line indicates the exactcalculation while the dashed line is the approximation of equation (9).Notice that the approximation is best for the L₁ stack and reasonablefor the L₀ stack.

In FIG. 7 the maximum allowed k value for the recording layer of L₁ isshown as a function of the recording layer thickness d_(L1) for variousvalues of the R_(min).

In FIG. 8 the special case where R_(min)=0.18 is drawn separately whereit the area with allowed k-values has been hatched.

In FIG. 9 the same is done and the graph for BD has been added as acomparison.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention a dual-stack optical data storage medium forrecording using a focused radiation beam having a wavelength λ isdescribed. The beam enters through an entrance face of the medium duringrecording. The medium comprises at least one substrate with present on aside thereof a first recording stack named L₀, comprising a recordabletype L₀ recording layer, said first recording stack L₀ having an opticalreflection value R_(L0) and an optical absorption value A_(L0) at thewavelength λ, and a second recording stack named L₁ comprising arecordable type L₁ recording layer, said second recording stack L₁having an optical reflection value R_(L1) and an optical absorptionvalue A_(L1) at the wavelength λ, and a transparent spacer layersandwiched between the recording stacks. By fulfilling the formulaA_(L1)≦1−R_(min)−√(R_(min)/R_(L0)) in which formula R_(min) is theminimum required effective optical reflection value for each recordingstack full compatibility is achieved with a read only (ROM) version ofthe medium.

1. A dual-stack optical data storage medium (10) for recording using afocused radiation beam (9) having a wavelength λ and entering through anentrance face (8) of the medium (10) during recording, comprising: atleast one substrate (1, 7) with present on a side thereof: a firstrecording stack (2) named L₀, comprising a recordable type L₀ recordinglayer (3), said first recording stack L₀ having an optical reflectionvalue R_(L0) and an optical absorption value A_(L0) at the wavelength λ,a second recording stack (5) named L₁ comprising a recordable type L₁recording layer (6), said second recording stack L₁ having an opticalreflection value R_(L1) and an optical absorption value A_(L1) at thewavelength λ, said second recording stack being present closer to theentrance face than the first recording stack, a transparent spacer layer(4) sandwiched between the recording stacks (2, 5), said transparentspacer layer (4) having a thickness substantially larger than the depthof focus of the focused radiation beam (9), characterized in thatA_(L1)≦1−R_(min)−√(R_(min)/R_(L0)) in which formula R_(min) is theminimum required effective optical reflection value for each recordingstack.
 2. A dual-stack optical data storage medium (10) according toclaim 1, wherein A_(L1)≦A_(L0).
 3. A dual-stack optical data storagemedium (10) according to claim 2, wherein1.5A _(L1) ≦A _(L0)≦2.5A _(L1).
 4. A dual-stack optical data storagemedium (10) according to claim 1, wherein the recordable type L₁recording layer (6) has a complex refractive indexñ_(L1λ)=n_(L1λ)−i*k_(L1λ) at the wavelength λ and has a thickness d_(L1)and the following formula is fulfilled:k _(L1λ) ≦{λ*ln[1/(R _(min)+√(R _(min)))]}/(4π*d _(L1))
 5. A dual-stackoptical data storage medium (30) according to claim 4, wherein therecordable type L₀ recording layer (3) has a complex refractive indexñ_(L0λ)=n_(L0λ)−i*k_(L0λ) at the wavelength λ and has a thickness d_(L0)and the following formula is fulfilled:k _(L0λ) ≦{λ*ln[1/(R _(min)+√(R _(min)))]}/(4π*d _(L0)).
 6. A dual-stackoptical data storage medium (10) according to claim 5, wherein λ isabout 655 nm, R_(min)=0.18.
 7. A dual-stack optical data storage medium(10) according to claim 5, wherein λ is about 405 nm, R_(min)=0.04.
 8. Adual-stack optical data storage medium (10) according to claim 5,wherein λ is about 405 nm, R_(min)=0.12.
 9. Use of an optical datastorage medium (10) as claimed in claim 1 for dual stack recording withan effective optical reflectivity level of more than R_(min) of both thefirst recording stack L₀ and the second recording stack L₁, R_(min)being the minimum required reflectivity level.